Some optical systems rely on ambient light from their surroundings to provide sufficient illumination for inspection, metrology, or any other type of sample analysis. Other optical systems require specialized illumination systems to project illumination within constraints such as, but not limited to, angle, area, wavelength, and/or intensity. Reticle inspection typically requires irradiance patterns directed at a reticle plane with specified numerical aperture and cone orientation. As such, Illumination systems for reticle inspection are among the most specialized illumination systems.
The extent of the irradiance pattern is also carefully controlled for reticle inspection, and irradiance uniformity at the reticle is among the most sensitive requirements. Present and past reticle inspection systems have relied upon a number of different illumination systems. However, extreme ultraviolet (EUV) reticle inspection presents problems that are not fulfilled by the current art. Some existing illumination systems utilize deep ultraviolet (DUV) wavelengths. Accordingly, the size and type of defects that can be detected and reviewed are limited.
Single source illumination systems are unable to provide illumination with sufficient brightness for EUV lithography, and multiple source illumination systems tend to exceed desired size restrictions and/or fail to provide the required uniformity. Various designs and concepts can be found in the following references: U.S. Pat. Nos. 6,396,068, 6,861,656, 7,183,565, US Patent Application No. 2007/0181834, and non-patent literature: Proc. OF SPIE Vol. 6517 56170J-1—Development of optics for EUV lithograph tools, published by NIKON Corporation. The foregoing are all incorporated by reference in their entirety as if fully set forth herein. In light of the foregoing systems and methods, there clearly exists a need for a cohesive and competitive illuminator layout that is compatible with plan-of-record optics design and also meets all known system throughput, footprint, cost, manufacturability, uniformity, size, and debris mitigation requirements.